Flexoelectricity-stabilized ferroelectric phase with enhanced reliability in ultrathin La:HfO2 films

作者：Peijie Jiao, Hao Cheng, Jiayi Li, Hongying Chen, Zhiyu Liu, Zhongnan Xi, Wen-Juan Ding, Xingyue Ma, Jian Wang, Ningchong Zheng, Yuefeng Nie, Yu Deng, L. Bellaïche, Yurong Yang, Di Wu · 发表于：Applied Physics Reviews · 年份：2023 · DOI：10.1063/5.0144958 · 被引用次数：22 · 研究领域：Ferroelectric and Negative Capacitance Devices、Cardiac Structural Anomalies and Repair、MXene and MAX Phase Materials

Doped HfO2 thin films exhibit robust ferroelectric properties even for nanometric thicknesses, are compatible with current Si technology, and thus have great potential for the revival of integrated ferroelectrics. Phase control and reliability are core issues for their applications. Here, we show that, in (111)-oriented 5%La:HfO2 (HLO) epitaxial thin films deposited on (La0.3Sr0.7)(Al0.65Ta0.35)O3 substrates, the flexoelectric effect, arising from the strain gradient along the film's normal, induces a rhombohedral distortion in the otherwise Pca21 orthorhombic structure. Density functional calculations reveal that the distorted structure is indeed more stable than the pure Pca21 structure, when applying an electric field mimicking the flexoelectric field. This rhombohedral distortion greatly improves the fatigue endurance of HLO thin films by further stabilizing the metastable ferroelectric phase against the transition to the thermodynamically stable non-polar monoclinic phase during repetitive cycling. Our results demonstrate that the flexoelectric effect, though negligibly weak in bulk, is crucial to optimize the structure and properties of doped HfO2 thin films with nanometric thicknesses for integrated ferroelectric applications.